The present invention relates to electrically conductive window glass, and more particularly to a control system for monitoring the condition of the window glass and interrupting power applied thereto in case of glass fracture.
A wide variety of electrically conductive windows have been developed which can be rapidly and effectively defogged and/or deiced. These windows are gaining wide-spread popularity in the automotive field where rapid defogging and deicing is not only a convenience, but a safety feature greatly enhancing the safe operation of the vehicle. For example, one particularly effective electrically conductive windshield is disclosed in U.S. patent application Ser. No. 438,789, filed Nov. 3, 1982, by Bitter, entitled PROCESS FOR MAKING A CONDUCTIVELY COATED GLASS MEMBER AND THE PRODUCT THEREOF, and assigned to the assignee of the present application. This windshield includes a conductive thin film, and more particularly a coating of indium tin oxide, on laminated glass. Electric current is conducted through the coating to resistively heat the glass and thereby defog and deice the windshield.
Typically, electrically conductive windshields draw 5 to 15, and even 30, amps of electric current at 20 to 60 volts during heating, possibly leading to several problems if not carefully controlled. First, if the temperature of the windshield becomes excessive, the windshield could possibly delaminate upon melting of the PVB lamination material. In the Bitter windshield, delamination could occur at approximately 150.degree. to 220.degree. F. Overheating also reduces the ability of the windshield to absorb in an accident the energy of impact of a person hitting the windshield but not penetrating the windshield. Such overheating therefore impairs the intended safety features of a conventional "safety" windshield. At least one prior artisan includes a temperature sensor on the windshield to detect an overtemperature condition and interrupt power applied to the windshield if the temperature becomes excessive. An example of this control is disclosed in U.S. Pat. No. 3,790,745, issued Feb. 5, 1974, to Levin, and entitled TEMPERATURE CONTROL OF ELECTRICALLY HEATABLE WINDOW.
Another possible problem arising from the currents and voltages involved with electrically heated windshields is the potential electric shock to an individual striking the windshield, for example in an accident. If a vehicle occupant strikes and fractures the windshield, it is possible that the occupant will engage the electrical potential and receive injury causing shock. This possibility, though remote, is of concern in commercial deployment of the product.
Yet another possible problem arising from this applied power is the potential of local ignition and flame that might occur if the electrically heated windshield breaks and sustained power continues to be applied to the windshield for a considerable time after the impact. Highly concentrated and sustained currents could cause ignition of the PVB lamination material. This may be a safety concern.
In an attempt to monitor this glass breakage, prior artisans have developed frangible bus bars secured to the glass. Examples are illustrated in U.S. Pat. No. 3,892,947, issued July 1, 1975, to Strengholt, entitled ELECTRICALLY HEATED PANEL WITH ANTI-SHOCK CONDUCTIVE STRIPS; and U.S. Pat. No. 3,524,920, issued Aug. 18, 1970, to Stromquist et al, entitled CIRCUIT BREAKER FOR CONDUCTIVE-COATED GLASS. Although these structures will adequately detect breakage when the bus bar is fractured, these structures do not detect relatively small centrally located window fractures or cracks, as might occur in striking a windshield. Further, these devices are not capable of detecting cracks in the conductive coating where the glass itself is not broken to the edge.
A security system sold under the mark STRATOBEL by Glaverbel of Brussels, Belgium, detects breakage of an electrically conductive window glass by monitoring the resistance of the electrically conductive window element. So long as the glass is unbroken, the resistance of the element remains constant. However, when the panel is broken, the resistance of the element changes to sound an alarm. However, it is believed that this system does not disrupt the flow of current to the glass.